Relative Dating: Which Came First?

What is relative dating? It is determining the age of a rock or fossils based upon the rocks or fossils around the fossil or rock in question. Example: If you go visit a preschool, even at 16 to 18 years of age, the kids at the preschool will think you are old. In this case your age, relative to your surroundings, is old. If you go visit a nursing home, the people there generally being elderly, will think you are just “babies” because you are so much younger than they are. In this case your age, relative to your surroundings, is young. RELATIVE DATING DOES NOT ALWAYS DETERMINE A NUMERICAL AGE!

STRATIGRAPHY Determining the order of geologic events by studying the rock layers is called Stratigraphy. Strata - a single bed of sedimentary rock, generally consisting of one kind of matter representing continuous deposition.

Laws of Relative Dating Law of Superposition: When sedimentary rock layers are deposited, younger layers are on top of older deposits.

The Law of Superposition For Sedimentary rocks, the oldest layers are at the bottom, and the youngest layers are at the top if the rock layer is undisturbed.

Notice the different color bands that indicate different layers of rock.

Which layer is the oldest according to superposition. How can you tell Which layer is the oldest according to superposition? How can you tell? Which is the youngest? Youngest Oldest

Rules of Relative Dating 2. Law of Original Horizontality: Sedimentary rock layers are deposited horizontally. If they are tilted, folded, or broken, it happened later. After Before

Law of Original Horizontality http://ageofrocks.org/2014/12/15/is-folded-sedimentary-rock-evidence-for-a-young-earth/

Rules of Relative Dating 3. Law of Cross-Cutting Relationships: If an igneous intrusion or a fault cuts through existing rocks, the intrusion/fault is YOUNGER than the rock it cuts through

Law of Crosscutting Relationships The intrusion or cross- cutting feature is the youngest thing present since the other layers had to be present before it could cut across them.

The same goes for Inclusions:

Inclusions An Inclusion of a foreign body of rock in an igneous rock is called a Xenolith. Conglomerate is a sedimentary rock formed when a matrix binds smooth pebbles (inclusions) together.

Exceptions: The law has to be applied with care, however, because some rocks contain rocky objects that develop after the rock formed. Perfectly formed crystals and lumpy objects known as concretions can be deposited from groundwater as it flows through sedimentary rocks.  Bismuth geode http://flickrhivemind.net/blackmagic.cgi http://i.imgur.com/IN4Iud9.jpg

Rules of Relative Dating 4. Law of Faunal Succession: helps correlate rocks across large distances by comparing fossils. The idea is that rocks of similar ages contain fossils of similar types, some of which do not appear in any other layer. The older fossils appear in the bottom while younger ones appear in the top

Index Fossils Some species of organisms only lived for a short period of time before they became extinct. They must be found over a widespread area and must be abundant. Trilobites were common during the Paleozoic Era (540 to 245 mya); about half of the Paleozoic fossils are trilobites. They evolved at the beginning of the Paleozoic Era and went extinct during the late Permian period (248 million years ago). 

Examples of index fossils include: Ammonites were common during the Mesozoic Era (245 to 65 mya), They were not found after the Cretaceous period, as they went extinct during the K-T extinction (65 mya). Graptolites (widespread colonial marine hemichordates) that lived from the Cambrian period (roughly 540 to 505 million years ago) to the early to mid-Carboniferous (360 to 320 million years ago). Trilobites were common during the Paleozoic Era (540 to 245 mya); about half of the Paleozoic fossils are trilobites. They evolved at the beginning of the Paleozoic Era and went extinct during the late Permian period (248 million years ago).   

Fossil B lived 10 million years ago – 2 million years ago. Fossil A lived 20 million years ago- 8 million years ago. What do you know about the age of the rock containing both fossils?

Unconformities are buried eroded surfaces which represent gaps in geologic time. Unconformities form when land above sea level is eroded and sinks. Sediment is then deposited over the eroded surface. The three types of unconformities are: 1) Angular Unconformity 2) Disconformity 3) Nonconformity

Angular Unconformity Sedimentary rock deposited horizontally over tilted, eroded layers

Disconformities A surface of erosion or non-deposition between parallel sedimentary rocks of different ages

Disconformity Rock layers are deposited horizontally over eroded, horizontal layers. May show a large gap in Geologic time. Sinking

Nonconformity Sedimentary strata deposited over eroded igneous or metamorphic layer sinking

3 types of unconformities are seen